In nuclear reactors, particularly of the pressurized water type, the control rods are suspended from suspension rods which pass into guide tubes fixed to the cover of the reactor and which are themselves closed by a cover so as to maintain the entire vessel under pressure, each tube being associated with mechanisms which, by acting on the suspension rod, enable the corresponding control rod fixed to the end of the suspension rod to be raised or lowered.
It is necessary to be able to watch the movement of each control rod constantly so as to be able to determine exactly the position of the rod and consequently of the absorbent spindle constituting the control rod in the fuel assembly.
The logging of the position of the control rod or of the suspension rod which supports it must obviously be effected without contact since it is done during the operation of the reactor.
Various types of apparatus have been used until now to determine the position of the control rods. It has been proposed, for example, to use a series of electromagnetic detectors arranged outside the housing of the control mechanism for the movement of the control rod, in association with a magnetic part of the rod. Thus, the movement of the rod modifies the inductive coupling between the windings of the detectors and generates an output voltage representing the position of the rod. Such a device has the drawback of being relatively bulky since the electromagnetic detectors must be arranged over the whole extent of the movement of the rod. In addition, the position of the rod cannot be determined with great accuracy.
To determine the position of a movable element without contact, particularly in a nuclear reactor, it has also been proposed to use the measurement of the variation in amplitude of an ultrasonic signal after its reflection on the member the movement of which it is desired to estimate. However, due to the fact that the propagation speed of an ultrasonic wave varies according to temperature, these methods are only good for a constant temperature or indeed must integrate a correction phase according to the temperature of the medium, which assumes an exact knowledge of the latter and introduces an additional error factor.